Finally fixed the login issue myself. You can too, if you're tech savy.
I've been dealing with this login failing ... forever loading but never actually successfully logging in issue for about 2 months now. During this time I've only been able to log in on one particular PC I have. The browser (Firefox) version I use between them is the same but I can only log in on one of them. I finally realized that it had to come down to the only difference between the 2 PCs, which is the version of Windows. The working PC has a more recent version of Windows installed.
In a nutshell, it seems they're checking the OS (Operating System) version your device/PC is running on. I don't know why but they seem to be restricting logins to Windows 8.1+. I don't use mobile so I can't say for sure, but it's likely that they are restricting older versions of Android or IOS from logging in as well even if the app otherwise works fine.
So the fix. Modify (spoof) your browser user-agent to make it seem like you are on a more recent version of Windows. A newer browser version/update isn't enough. Searching online can provide all the details. It's not difficult and they do have addons that can accomplish this relatively easily. After spoofing the UA, I was immediately able to log in. Finally!